In particular in the field of adhesives, sealants, coating agents and molding compounds as well as non-aqueous based oil-drilling fluids and the like, it is necessary to customize the rheological properties of such systems, primarily through adjusting the consistency by the selection of binding agents, solvents and the concentration of pigments and/or fillers. However, in many cases an adjustment to the desired consistency with the help of the aforementioned components is not sufficient. In such cases, the addition of so-called rheology additives (i. e. rheological additives) is required. The effect of this can be lowering the viscosity for the purpose of better processability or an increase in viscosity, which in the present invention is referred to as “thickening.”
Many different organic or inorganic thickeners are described for the purposes cited.
Commonly used in aqueous systems are cellulose ether, starch, natural hydrocolloids, synthetic biopolymers, polyacrylate thickeners, associative thickening agents based on hydrophobically-modified polymers such as polyethers, ether urethanes, polyacrylamides and alkaline activated acrylate emulsions or water swellable inorganic thickeners.
Typical rheology additives for non-aqueous systems include, in addition to organic thickeners such as waxes and thixotropic resins, inorganic thickeners such as, for example, magnesium oxide and magnesium hydroxide or amorphous silica and layered silicates.
Among layered silicates, clay materials are often used as thickeners; among these, organically modified clay materials (also known as organoclays) are used in the art.
Organoclays are widely used as thickeners, more specifically thixotropes, for organic solvents or organic coatings systems like paints and varnishes and in composites as well. They are typically used in an additive amount, i.e. an amount of 10% by weight or less to improve the application properties of liquid organic systems. Such thickened organoclay containing liquid organic systems show thixotropic or pseudoplastic behavior. This improves the storage stability of organic systems. It prevents the demixing and especially the sedimentation of pigments and fillers in the system during storage. It keeps the organic systems homogeneous.
A further purpose for which the organoclays are used is to increase the sag resistance of liquid organic systems. Typically paints which are applied to vertical surfaces tend to run down and cause drop formation on the substrate. The paints can only be applied in a certain limited film thickness before they start running down or start dropping. This is usually called “sagging”. Organoclays can reduce sagging by increasing the viscosity of the system. The increase in viscosity can be regarded as a gelling of the systems within a time frame of seconds to minutes.
The way the organoclays are doing this is mostly referred to as a house of cards structure. The organoclay is made out of clay minerals in form of platelets or in form of needle-like pseudo laminar structures. The clay platelets at rest are supposed to orientate themselves in a shape similar to a house of cards. This house of cards has a certain stability which provides the gelling and anti-sagging properties. But when moved during application by painting, spraying and the like, this house of cards breaks down and only low apparent viscosity is visible during application. After application, when there is no strong external force on the liquid, this house of cards recovers within seconds to minutes and gives viscosity in form of gelling at low shear forces. This can be measured by means of a rheometer especially when the viscosity is monitored at low shear forces, i. e. low speed of the rotating part of the rheometer. That means organoclays do not increase the apparent viscosity of the liquid system too much during application, but gel formation occurs very quickly after the application due to the strong low shear viscosity input.
Organoclays typically are made from inorganic clays. Natural clay is usually only badly compatible with solvents. Typically it is hydrophilic and mainly suspendable in water, less in most organic solvents. Therefore the surface of the clay mineral has to be rendered hydrophobic. This is done by an organic coating of the clay surface. The clay mineral is typically anionically charged. The most common method is to treat the anionic clay with cationic surfactants, namely quaternary ammonium salts. Both educts are typically reacted in water. The quaternary ammonium goes to anionic charges on the clay surface and gives a strong bonding to these charges. The clay surface is rendered hydrophobic at that point and the clay loses water solubility. The thus modified clay is usually suspendable in organic solvents, but less in water.
WO-A-97/17398 and U.S. Pat. No. 6,036,765 disclose mixtures of clays that were surface-treated with quaternary alkyl-ammonium salts and which comprise about 50 to 95% by weight, in relation to the layered clay mixture, of a clay mineral selected from the group consisting of sepiolite and/or palygorskite and contain less than 50% by weight, in relation to the layered clay mixture, of at least one smectite. The smectite or smectites can preferably be selected from the group consisting of hectorite, montmorillonite, bentonite, beidelite, saponite, stevensite and mixtures thereof. None of both publications describes the use of hormite clays having an extraordinary low content of specific metal oxides namely aluminum oxide and/or iron oxide in the manufacture of the therein produced rheology agents.
U.S. Pat. No. 8,389,447 B2 describes organoclay additives to provide oil well drilling fluids with less temperature dependent rheological properties. The additives disclosed therein are based on a synergistic combination of a hectorite organoclay composition and an attapulgite organoclay composition. Each of both clays was treated with at least two different quaternary ammonium salts one of which has to be an alkoxylated quaternary ammonium salt before the thus treated clays were mixed.
WO 2015/138407 A1 also describes mixtures of preformed organophilic modified attapulgite and/or sepiolite clays with preformed organophilic modified bentonite clay for use in oil-based drilling fluid base compositions. However, in contrast to WO-A-97/17398 and U.S. Pat. No. 6,036,765 the most preferred amount of the modified attapulgite/sepiolite in the mixture with the modified bentonite is very low and in the range of about 10% by weight, only.
The thickeners described in the prior art seem to have an average to good performance in the systems they were used in. However there is still a need to provide an improved process of manufacturing thickeners. Using the thus produced thickeners should lead to very reliable and mostly significantly increased thickening effects particularly in specific application systems such as epoxy resin formulations which are widely used, e. g. in coatings, sealants and adhesives.
In the storage form of epoxy resin containing liquid compositions, hardeners are typically not contained. Typically one or more hardeners are mixed with or into such liquid compositions only shortly before the application to form a ready-to-use formulation. After mixing the hardener with or into the liquid epoxy resin containing compositions a curing reaction between epoxy resin and hardener occurs. This reaction is usually exothermic, resulting in an increase of temperature. The initial increase of temperature is usually associated with a decrease of viscosity of the formulation. However at a later stage, when curing has proceeded, viscosity will start to increase again, due to the formation of larger molecules formed in the reaction between epoxy resin and hardener. Therefore the initial decrease in viscosity after the start of the reaction of the epoxy resin with its hardener demands an extraordinarily high performance of the thickener used. By any means, it needs to be avoided that the initial decrease of viscosity of the ready-to-use formulation leads to undesired sagging and the like. With other words the increase in viscosity by adding the thickener to the hardener-free storage form should already be excellent.
In summary, it was the aim of the inventors of the present invention to provide an improved process of manufacturing thickeners and the use of the thus produced thickeners in liquid epoxy resin formulation. It was a further aim of the inventors to provide a very reliable process of producing an organoclay based rheology additive with a higher performance compared to structurally related organoclay additives known from the state of the art.